A widely used form of analog-to-digital converter utilizes a plurality of differential amplifier circuits, each referenced to a different voltage within the range of the analog voltage input. See for example U.S. Pat. No. 3.964,064 issued June 15, 1976, to R. L. Brandao, et al. As each differential amplifier involves two transistors or other amplifying devices with associated circuitry, the cost of such an analog-to-digital converter is relatively expensive.
Analog-to-digital converters are used in video display devices which display different levels of brightness depending on the output of the converter. Typically one of the levels to be displaced is manifested as a dark screen. In some prior art systems the display drive circuitry is permanently clamped at an ideal threshold of visibility, that is, a point at which the screen is on the borderline between darkness and minimal brightness. It is then altered by the output from the analog-to-digital converter for different levels of brightness. Unfortunately the display device may not in reality be set precisely to the threshold of visibility due to, for example, changes in environmental conditions. As a result desirably dark portions of the screen may appear undesirably at some level of brightness.
An alternative prior art solution consists of setting the drive signal threshold well below the threshold of visibility. Then appropriate signals from the analog-to-digital converter cause various brightness levels to appear on the screen including truly dark areas. Such a system appears to be an improvement over the previously mentioned system which is set at the threshold of visibility, but the improvement is not found to exist in practice due to the presence of a brightness control for setting the overall brightness as desired by the viewer. As the brightness control is adjusted to raise the brightness level sufficiently above some average value, the brightest portions of the display reach some upper limit and desirably lesser brightness areas become undesirably as bright as the brightest areas. Also when the brightness control is adjusted to lower brightness level sufficiently below the average level, minimal brightness areas on the screen undesirably merge with the dark areas. That is, with the brightness control set sufficiently above or below some average value available resolution is lost in the presentation.